Power assist control capable of responding to feeling and sensibility when a man applies force to an object is performed. The power assist control outputs an assist force F or a speed F on the basis of an assist target force X detected by a force sensor 114 for detecting force applied to the object. The relationship between the assist target force X and the assist force F or the speed F to be outputted is F∝f(X) on the basis of a function f(X) having the assist target force X as a variable. The function f(X) is assumed to be a non-linear function which is an increasing function and whose derivative f′(X) is a decreasing function. The function f(X) is a (1/n)-th order function or a logarithmic function. Moreover, a function fup(X) when the assist target force X is increased is different from a function fdown(X) when the assist target force X is decreased.
